Tomcat is the reference implementation of the Java Server Pages(JSP) and Java Servlet specifications. This means it is the most standards compliant Java server available.
Despite Tomcat's popularity, it suffers from a common shortcoming among open source projects: lack of complete documentation. There is documentation distributed with Tomcat mirrored at jakarta.apache.org. However, there is also an open source effort to write a Tomcat book at tomcatbook.sourceforge.net